Phil. Trans. R. Soc. A (2012) 370, 5101–5108 doi:10.1098/rsta.2012.0097 Binary recombination of para- and ortho-H + 3 with electrons at low temperatures BY P. DOHNAL 1 , M. HEJDUK 1 , J. VARJU 1 , P. RUBOVI ˇ C 1, *, Š. ROU ˇ CKA 1 , T. KOTRÍK 1 , R. PLAŠIL 1 , R. JOHNSEN 2 AND J. GLOSÍK 1 1 Faculty of Mathematics and Physics, Department of Surface and Plasma Science, Charles University, Prague, Czech Republic 2 Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260, USA Results of an experimental study of binary recombination of para- and ortho-H + 3 ions with electrons are presented. Near-infrared cavity-ring-down absorption spectroscopy was used to probe the lowest rotational states of H + 3 ions in the temperature range of 77–200 K in an H + 3 -dominated afterglow plasma. By changing the para/ortho abundance ratio, we were able to obtain the binary recombination rate coefficients for pure para-H + 3 and ortho-H + 3 . The results are in good agreement with previous theoretical predictions. Keywords: H + 3 ; dissociative recombination; cavity ring down spectroscopy; afterglow plasma 1. Introduction The fundamental characteristics of the H + 3 dissociative recombination (DR) [1] have been the subject of much interest for both theoretical and experimental physicists [2]. The discrepancies between measurements of the binary dissociative reaction rate and the theoretical complexity of this seemingly simple reaction led to a great deal of fruitful research on this process. The history of H + 3 recombination studies has been adequately covered in several review articles [3–9]. Recently, both theory and experiment have converged to a value for the rate of this particular reaction. The theoretical treatment took a crucial leap forward in the understanding of the DR process after including the Jahn–Teller mechanism as the critical step in the initial electron-capture step of the DR reaction [10]. This resulted in a convergence with experimental DR data reported from ion storage rings where experimentalists had realized the impact of rotational excitation of the H + 3 ions on the DR reaction rate, especially with respect to the importance of the DR process in interstellar molecular clouds [11–13]. Final convergence between theory and the remaining important experimental techniques, stationary and flowing afterglow, was reached after recognizing that a *Author for correspondence (peter.rubovic@gmail.com). One contribution of 21 to a Theo Murphy Meeting Issue ‘Chemistry, astronomy and physics of H + 3 ’. This journal is © 2012 The Royal Society 5101